Telephone system



-' 1943- E. P. FAIRBAIR N' 2,3 1

TELEPHOfIE SYSTEM Filed Oct. 24, .1940

. INVENTOR 1-72 ATTORNEY Patented Mar. 16, 1943 TELEPHONE sYs'rEM EricPlayfair Fairbairn, Coventry, England, assignor to The General ElectricCompany Limited, London, England, a British company Application October24, 1940-, Serial No. 362,521v In Great Britain October 31, 1939 2Claims.

The present invention relates to telephone systems, and has for itsobject the provision of such a system having improved transmissioncharacteristics.

If a normal telephone connection as set up through one or more exchangesis considered, it will be seen that few conditions for best speechtransmission are fulfilled. For example, the impe'dances of thesubscribers instrument and line,

' and of the exchange speech bridges and junction lines if included inthe connection, are usually widely different from one another. Again, itis often necessary to connect low impedance subscribers lines witheither normal or high impedance lines or terminations during differentcalls.

,LfiClllCY from bad matching, echoes from reflection at junction pointsand possibly phase dis- The former condition causes loss of efplacement,owing to the inductive or capacitative nature of the reactances of thevarious components in circuit. The latter prevents adequate and completematching of subscribers instru- Tments to the lines under allconditions, this resulting normally in an unnecessary amount of residualside tone.

According to the invention, a telephone system is provided in which allelements of a connection, that is the lines, trunk lines with feedbridges, and any other exchange equipment ina connection are faced withtheir iterative impedance, each substation circuit forming the closingimpedance of a subscribers line which is therefore characteristicimpedance of that line,

which is the same as the characteristic impedance of a connection as awhole. Tothis end, we provlde each telephone instrument with anefficient transformer, preferably with a closed or partially closed ironcircuit, and a linebalancing network so that the combination providesthe said characteristic impedance to the line. In such an instrumentside tone can, if necessary, be substantially eliminated.

A correcting network is also provided for each subscriber's line, thisbeing adjustable so that a vsingle type of network may be manufacturedand then fitted on any type of line. For overhead lines these networkspreferably comprise resist acteristic desirableover"the frequency bandemployed for speech transmission. Alternatively distributed loading ofknown type may be employed.

It is also arranged that the speech bridge or like transducer employedin any selector or cord circuit in the exchange acts as a filter elementhaving the necessary characteristic impedance over the frequency bandfound necessary in practice. This may be accompl'shedieither byproviding filter components i dition to those normally found in aspeechbridge, or by adjusting the components to 'such values'that theythemselves form the filter.

Junction impedances are'brought to the correct value by the use ofsuitable termination transformers, phase correcting networks'or thelike, and by the employmentlof repeaters or amplifiers at suitable:points. f

Owing to the non-reflective characteristics of an exchange network ofthis'type speech quality is appreciably improved, In addition, as eachsubscribers instrument always works into the same impedance, it'ispossible to provide a matching networkwhich'balances'the external lineconbe eliminated over' the "chosenw'frequency band; I Similarly,'bothway loudspeaklngtelephones may ditions exactly,:so that side tonecan if necessary be provided between subscribers ifa'coustic couplingbetweeneach subscribers loudspeaker and microphone is "reduced-to asuitably'low value. 'It should be noted'that since the terminalimpedance of a junction of, anyappreciable length is substantiallyindependent vofthat length, it would bepossibleto connecta telephonesystem gaccordingto the invention with an adjacent un- "corrected systemviajunction lines without loss of any of the facilities; mentioned.

Reference should now-be made to the accompanying drawing" which showsparticular embodiments of the inventionfbut it will be ap preciated bythose 'skilled'in the art that it is not necessarily limited to theexamples shown. In the drawing, Figure 1 is an explanatory diagram ofsubscribers instrument circuit, Figure 2 is a complete schematic ofsubscriber's instrument circuit, Figure 3 showsa typical speech bridgeincluding relaysandFigu're 4 is Figure 3 redrawn as a filter ele nt';

' Referring to Fig g land. 2,.it will be seen that these consistessentially fhybrid type transformer Thaving tidings T T2, T3 and-T4 Thetransformer is connec ed to line wires-and line balance each ofimpedance Z, to a microphone G having impedance ZI, and to a receiverhaving an impedance-Z2; Assuming that Ti and in portions of the circuitas necessary.

T2 have an equal number of turns, Zl is made equal to Z/2. By adjustingthe turns of T3 and T4, the impedance Z2 can be accurately matched. Ifthe number of turns on winding TI is n, then the number of turns on coilT3 must be For speech voltages-developed across Z! a signal is passedout on to the line, the power being equally divided between the balanceand the line, i. e. a 3 db. transmission loss and no voltage isdeveloped across Z2. This is independent of frequency. For reception,assumingthat the line balance is exact and the transformer perfect, thepower is divided equally between the receiver and transmitter.

The present invention uses the above method of design in the followingmanner. The hybrid coil may have a complete, nearly complete or openiron circuit. Both the line and balance are given a 600 ohms resistiveimpedance. Z2 is replaced by' a telephone receiver. If the receiver doesnot present a constant resistive load, then a simple correcting networkcan be used in conjunction with it. Zl can be replaced by a highresistance microphone, either of impedance placed directly in place ofZl, or of different resistance. In the latter case an additionalinduction coil or transformer is required to be used in conjunction withthe microphone for matching purposes.

The circuitin Figure 2 indicates one embodiment of the invention so faras it relates to subscribers' instruments. In. this figure, M is amicrophone corresponding to Zl and G in Figure 1. As the impedance ofthe microphone is different from ZI, an auto-transformer AT is employedto change its effect on the circuit to the required value. A receiver Rand the network C2, C3 and R3 correspond to the impedance Z2,

both in magnitude and phase angle. The other portions of the circuitsuch as Cl are means em-. ployed to control the direct current derivedfrom the exchange to energise the microphone and for signallingpurposes. Hook-switch and dial contacts can be provided if required andinserted If the microphone talking resistance with the required value offeed current is RI, then a step-up from RI to 2/2 (i. e. 300 ohms) isrequired. This is' achieved by. means of auto-transformer AT in knownmanner, the ratio of the turns'on windings ATI and ATZ being in theratio This transformer can be made with a complete, nearly complete oropen iron circuit, The battery feed is localised to the transmittercircuit by means of the blocking condenser Cl. A high resistancemicrophone of 200 or 300 ohms can be employed, requiring'only a feedcurrent of 30 milli-amperes cr'20 milli amperes respectively, the speechoutput of which can be' arranged to over-compensate for the transmissionloss referred to above. The lower feed currents simplify the hybrid coildesign.

If the circuit is ideally balanced, there will be no side-tone orvoltage developed in the receiver during transmission. In practice,however, some side tone is desirable, the actual amount depending tosome extent on the extraneous noise. The amount of side-tone can becontrolled by upsetting the balance of the hybrid coil by means ofconnection of a resistance in shunt of winding TI of the transformer.The circuit also includes one form of correcting network for a receiver,comprising a resistance R3 and the condensers C2 and C3. This, togetherwith the receiver, can be made to present to the hybrid coil a 400 ohmsimpedance sensibly resistive and independent of frequency.

Referring now to Figures 3 and 4, Figure 3 shows a speech bridge of atype normally found in cord circuits or selectors. The speech bridgeconsists of two condensers KI and K2, and two relays SI and S2.Hitherto, no attempt has been made to design this combination as afilter,

one of the reasons being the very low value of the inductance-resistanceratio (or Q) of the relay coils. It is found that by employing alaminated core construction and suitably choosing the number of turnsand the condenser capacities, a speech bridge can be provided having thecharacteristics of a filter of the type shown in Figure 4. For example,if each relay has a phase angle of degrees and the combined inductanceof its coils is .476 henry, then each of the condensers KI and K2 shouldhave a value of 1.32 micro-farads in order to provide a highpass filterhaving a cut-off frequency of 200 cycles, the network havinga'characteristic impedance of 600 ohms. The battery connections to therelays are similar tothose shown in Figure 3. As however the relayscannot be made perfect inductances, some slight variation of thesevalues may be necessary to secure best results in practice. Suchadjustments will be obvious to those skilled in the art.

It is preferable to provide, inaddition to the inductances shown, acurrent stabilizing device such as a barretter in series with the linecurrent circuit as this enables the constants of the variouscurrent-carrying inductances to be more easily obtained and maintained.

Where both long and short lines are to be connected in series e. g.junction lines and subscribers lines, it is preferable that thecharacteristic impedance of each line should be the same as that ofevery other line to which it is connected. This can be assured by theprovision of matching networks or loading of known type.

The telephone circuit shown in Figure 2 can, if necessary, be modifiedso as to provide loud speaking facilitiesin fact, the system as a wholeis eminently suitable for use in connection with such loud-speakingdevices. For this purpose, the michrophone M and auto-transformer AT areremoved and the output stage of a microphone-amplifier is substituted.Similarly, the receiver R and its input matching circuit are removed andreplaced by the input stage of a loud-speaker amplifier. If necessary,these two amplifiers can be made into one, and suitable switching isthen employed so as to enable the amplifier to perform its dualfunctions.

It may occasionally be necessary to tap off small quantities of speechcurrent, as for example to an operators or monitors circuit. Such branchcircuits are preferably made of such a high impedance, as for example byincluding a valve amplifier, that the characteristic impedance of theconnection as a whole is substantially unaffected. r

I claim:

1. In a telephone system, subscribers substae tion circuits, linesconnecting said circuits to the exchange, a plurality of connectingcircuits in the exchange, battery feeding bridges in each connectingcircuit, means including said connecting circuits for connecting oneline to another, a microphone in a bridge circuit at each substationcircuit together with a balancing network on one side of the bridge tobalance the impedance of the connecting means or line connected to theother side thereof, a receiver circuit connected in anti side tonerelation to said microphone, a corrective network connected in saidreceiver circuit to equalize the impedance thereof over the desiredfrequency range, the impedance of said elements in the substationcircuit adjusted so as to match the characteristic impedance of thesubscribers line when viewed from the line, the impedance of the feedingbridges of each connecting circuit adjusted to present the samecharacteristic impedance as the subscribers line as a whole, wherebyeach line and each connecting circuit of a connection faces itsiterative impedance when the same are connected together.

2. In a telephone system, a subscribers line, a plurality of connectingcircuits each having a feeding bridge, each feeding bridge comprising animpedance adjusting filter adjusted to provide the same characteristicimpedance to each circuit, a sub-station circuit connected to each line,a transformer in the substation circuit, a microphone in bridge of theline at each substation, an impedance bridge at each substation havingan impedance equal to the characteristic impedance of theline to whichit is connected, the impedance of the transformer, the microphone bridgeand the impedance bridge of each substtion being adjusted so that thesubstation circuit has a characteristic impedance matching thecharacteristic impedance of the line when viewed from the line, eachline and connecting circuit in the system having the same characteristicimpedance.

ERIC PLAYFAIR FAIRBAIRN.

